The present invention relates to a radio apparatus such as a portable radio.
Mobile communication apparatuses typified by portable telephones have been enthusiastically developed. These communication apparatuses are carried by users or mounted in cars and the like, and hence must be reduced in size and weight. For this reason, as parts constituting such a radio apparatus, monolithic integrated circuits, which are more suitable for reductions in size and weight than a conventional hybridization structure formed by connecting many parts, are strongly required. On the other hand, demands have arisen for reductions in the costs of radios as well as reductions in the sizes of parts. IC technology leads to reductions in the costs of radios.
Among these radio apparatuses, there is a conventional transceiver apparatus using the superheterodyne scheme, e.g., a transceiver apparatus using the TDD (Time Division Duplex) scheme of time-divisionally performing transmission and reception. There will now be described a conventional transceiver hereinafter.
Two quadrature baseband signals generated by a baseband signal generator are output from a baseband signal processor through some appropriate band limit filter. These two baseband signals are input to a quadrature modulator comprised of a multiplier and an adder to modulate a second local oscillation signal (frequency f.sub.LO2). At this time, the second local oscillation signal is divided into two quadrature local oscillation signals by a quadrature phase shifter. The signals are then input to the quadrature modulator. Since this modulated signal, i.e., intermediate frequency signal (IF signal), generally includes unwanted harmonics, the signal is input to an up-converter through a low-pass filter or bandpass filter. The up-converter multiplies a first local oscillation signal having a frequency f.sub.LO1 [Hz] and the modulated signal (IF signal) to generate a modulated signal having a frequency f.sub.LO1 +f.sub.LO2 [Hz] or f.sub.LO1 -f.sub.LO2 [Hz], i.e., radio frequency signal (RF signal). One of these signals is used as a desired wave, but the other signal is an unwanted image signal. If the RF signal having the frequency f.sub.LO1 +f.sub.LO2 [Hz] is used as a desired wave, the image signal having the frequency f.sub.LO1 -f.sub.LO2 [Hz] is removed by an image rejection bandpass filter. The desired wave is amplified to a predetermined power level by a power amplifier. The amplified wave is radiated from an antenna through an RF switch (TX/RX switch).
In a receiver unit, the received RF signal is input to a low-noise amplifier through an antenna, an RF switch, and a bandpass filter. The received RF signal amplified by the low-noise amplifier is input to a down-converter through an image rejection bandpass filter. The down-converter multiplies the received signal by a first local oscillation signal to frequency-convert the received signal into an IF signal. The IF signal is input to a quadrature demodulator made up of a divider and a multiplier through a bandpass filter. A quadrature local oscillation signal having the frequency f.sub.LO2 [Hz] is input to this quadrature demodulator as in the case with a transmitter unit. Outputs Ich(RX) and Qch(RX) from the quadrature demodulator are input to a baseband signal processor. As a result, the received signal is demodulated.
According to the above conventional radio apparatus, when a local oscillation signal is to be modulated by a baseband signal, the local oscillation signal is divided into two quadrature signals by the quadrature phase shifter. The two signals are input to the quadrature modulator. Since this modulated signal (IF signal) generally includes unwanted harmonics, the signal is input to the up-converter after these unwanted harmonics are removed. This up-converter multiplies the modulated signal (IF signal) by the first local oscillation signal having the frequency f.sub.LO1 [Hz] to generate a modulated signal (RF signal) having the frequency f.sub.LO1 +f.sub.LO2 [Hz] or f.sub.LO1 -f.sub.LO2 [Hz]. One of these signals is used as a desired wave, but the other signal is an unnecessary image signal. To remove this unnecessary image signal, this signal is processed by the image removing filter. This image filter hinders reductions in the size and cost of the radio apparatus.
It is, therefore, an object of the present invention to provide a radio apparatus which requires no external image removing filter and can achieve reductions in size and cost.